For a theoretical discussion of stack effect, see Chapter 16 in the 2017 ASHRAE Handbook—Fundamentals. That chapter describes calculation of the theoretical total stack effect for temperature differences between the inside and outside of the building. It also points out that every building has a neutral pressure level (NPL): the point at which interior and exterior pressures are equal at a given temperature differential. The location of the NPL is governed by the actual building, the permeability of its exterior wall, the internal partitions, and the construction and permeability of stairs and shafts, including the elevator shafts and shafts for ducts and pipes. Other factors include the air-conditioning systems: exhaust systems that extend through the entire height of the building tend to raise the NPL, thereby increasing the total pressure differential experienced at the base of the building. This also increases infiltration of outdoor air, which tends to lower the NPL, thus decreasing the total pressure differential experienced at the base of the building. Finally, wind pressure, which typically increases with elevations and is stronger at the upper floors of a building, also can shift the neutral plane, and should be considered as an additional pressure to stack effect when locating the neutral plane.

Figure 1 depicts airflow into and out of a building when the outdoor temperature is cold (stack effect) and hot (reverse stack effect). Not shown is the movement of air up or down in the building as a function of stack effect. Assuming there are no openings in the building, the NPL is the point in the building elevation where air neither enters nor leaves the building. Vertical movement of air in the building occurs at the paths of least resistance, including but not limited to shafts and stairs in the building as well as any other openings at the slab edge or in vertical piping sleeves that are less than totally sealed. Figure 1 also indicates that air movement into and out of the building increases as the distance from the NPL increases. Elevator shafts, especially ones that connect the top and bottom of a tall building (e.g., a fire lift), are likely paths of least resistance for airflow. The total theoretical pressure differential can be calculated for a building of a given height and at various differences in temperature between indoor and outdoor air.

The theoretical stack effect pressure gradient for alternative temperature differences and building heights is shown in Figure 2. The diagram shows the potential maximum differentials that can occur (which are significant), but these plotted values are based on an idealized building with no internal subdivisions in the form of slabs and partitions. The plot, therefore, includes no provisions for resistance to airflow in the building. Further, the outer wall’s permeability influences the values on the diagram and, as noted previously, the wind effect and operation of the building air-handling systems and fans also affect this theoretical value. Thus, the diagram should be considered an illustration of the possible magnitude of stack effect, not as an actual set of values for any building. The actual stack effect and location of the NPL in any building are difficult (if not in a practical sense impossible) to determine. A real building, especially a tall building, may have multiple neutral planes because of the effects of elevator and stair transfers. Each shaft section (e.g., low-rise elevator shaft) imparts its own stack effect and can create variations in the building pressure profile at the top and bottom of the shaft. Nevertheless, stack effect can be troublesome, and its possible effects must be recognized in the design documentation for a project.

Figure 1. Airflow from Stack Effect and Reverse Stack Effect (Ross 2004)

Figure 2. Theoretical Stack Effect Pressure Gradient for Various Building Heights at Alternative Temperature Differences (Ross 2004)

Stack effect in tall buildings often presents major problems:

Uncontrolled infiltration and ventilation is caused by climate, wind pressure, and stack effect; environmental factors associated with stack effect include wind pressure, stack pressure difference, airflow rate, outdoor and indoor temperature, building height, and building construction.

Wind creates a distribution of static pressure on the building envelope that depends on wind direction and velocity against the building envelope. The basic formula to determine this pressure can be expressed as

(1)

where

When using this equation, wind pressure is 0.1 in. of water at 15 mph on the windward side.

Air density varies with temperature. In cold weather, low-density air infiltrates the high-rise building and rises in the building’s vertical shafts as it warms, creating stack effect pressure. The basic stack effect theory is expressed as

(2)

where

When using Equation (2), the stack pressure is 1.1 in. of water for a 60-story building with –10°F OA temperature.

Once the wind pressure ΔP_W and stack pressure difference ΔP_s are calculated, total pressure ΔP_total can be found, based on indoor and outdoor pressure difference, and used to calculate the airflow rate:

(3)

Calculation Example. For the calculation examples, New York was selected because it has many tall buildings and a significant range between warm summer and cold winter temperatures (stack effect influences buildings differently at different temperatures). The following example investigates performance in both summer and winter conditions.

ASHRAE climate data were used (see Chapter 14 of the 2017 ASHRAE Handbook—Fundamentals) and show the winter and summer temperature and humidity levels, which can be used to calculate stack effect.

Table 1 gives the example parameters, and Figures 3 to 7 show various conditions. As shown in Figure 4, the biggest difference between internal and external pressure occurs in winter, when internal pressure increases along the building height; in summer, it decreases along the height. In addition, when the building gets taller, its NPL on the windward side rises: the extreme is for a building height of 2700 ft, for which the NPL on the windward side is almost on the top of the building.

Figure 3. Reduction in Ambient Temperature Over Height of Building in Cold Ambient Conditions

Figure 4. Windward, Internal, Leeward, and Stack Pressures during Winter

Figure 5. Reduction in Ambient Temperature over Height of Building in Warm Ambient Conditions

Figure 6. Windward, Internal, Leeward, and Stack Pressures during Summer

Figure 7. External Wind Speed as Function of Building Height at Standard Atmospheric Conditions

For the climate in New York, which is cold and dry in winter and warm and humid in summer, stack effect is much more intense than in warmer climates. Stack effect during cold outdoor conditions may cause problems, such as elevator doors not closing properly because of the pressure differential across the doors, causing the doors to stick in their guideways if the closing mechanism cannot overcome this friction.

Another difference for New York compared with other cities occurs in the wintertime, when NPL is slightly lower, or below the middle of the building. During winter in a 2700 ft building in New York, the NPL is slightly below 984 ft, which means the indoor air pressure is much higher at the upper level of the building than in many other cities. Therefore, for upper levels of the building, air exfiltration is much greater in New York than in other, warmer cities; the airflow rate is higher; and the building function is significantly affected. Stack effect must be addressed during design. Architects and engineers must pay close attention to solving the problems associated with stack effect, which are exacerbated in extremely cold climates.

During design, the architect and HVAC design engineer should take steps to minimize air leakage into or out of (and vertically within) the building. Although it is not possible to completely seal any building, this approach can help mitigate potential problems that could be caused by stack effect.

A tight building envelope and continuous curtainwall system, as well as isolation of vertical shafts from exterior environment through a minimum of two air barriers, are fundamental to protecting a building against uncontrolled air movement into and out of the building. A tight specification, testing, and careful monitoring during construction are required. Although most curtain walls are traditionally tested in the factory, increasingly more field tests (either spot tests or whole-building pressure tests) are specified in some buildings. A whole-building pressure test is difficult in tall buildings, and currently there is a limit on how many floors can be pressurized. Sectionalizing a tall building is required to perform a localized test.

Outdoor air infiltration points include building entry doors, doors that open to truck docks, outdoor air intake or exhaust louvers, construction overhangs with light fixtures, or other recessed items, located immediately above the ground level and are not properly sealed against leakage or provided with heat, and any small fissures in the exterior wall itself. Internally, the building allows air passage through fire stairs, elevator shafts, mechanical shafts for ducts and piping, and any other vertical penetrations for piping or conduit or at the edge of the floor slab at the exterior wall. All these are candidates for careful review to ensure, as much as possible, that the exterior wall is tight, all shafts are closed, and all penetrations sealed. Vestibules or airlocks can be provided for loading docks, with good door seals on the doors to and from the loading dock.

Entrances for tall buildings in cold and hot climates should use revolving doors. Doors of this type are balanced, with equal pressure in opposite directions on the panels on either side of the central pivot, making operation relatively simple and requiring no special effort to turn. Their gaskets also provide closure at all times. Give special consideration to hotel entries where people carry large luggage through: larger revolving doors are required to avoid people bypassing the revolving doors altogether.

Design and layout of sky lobbies in super- and megatall buildings should be carefully considered to isolate building elevator shafts and exit stairs from the exterior environment. A vestibule may be added at the elevators to provide a second air barrier in addition to the building envelope.

Two-door vestibules are acceptable for the loading dock, assuming the doors are properly spaced to allow them to be operated independently and with one door to the vestibule always closed, and sufficient heat is provided in the space between the doors. If properly spaced, simultaneous opening of both doors on either side of the vestibule can be controlled. However, two-door vestibules in cold climates are inadequate for personnel entry because, with large numbers of people entering the building at various times, both doors will be open simultaneously and significant quantities of unconditioned outdoor air can enter the building. In cold climates, revolving doors are strongly recommended at all points of personnel entry.

To control airflow into the elevator shaft, consider adding doors at the entry to the elevator banks. This creates an elevator vestibule on each floor that minimizes flow through open elevator doors.

Elevator shafts are also a problem because an air opening may be required at the top of the shaft. In many tall buildings, however, the elevator hoistways are not vented for smoke, using sprinkler heads instead. Alternatively, some jurisdictions accept the installation of a motorized damper on the hoistway vent; the damper is initiated by a smoke detector and opens immediately when smoke is sensed in the hoistway. All shafts, however, can be sealed in their vertical faces to minimize inflow that would travel vertically in the shaft to the openings at its top.

Elevator cars can act as pistons to increase the pressure in elevator hoistways ahead of the moving cab. Careful sequencing of elevator cabs, especially when multiple cabs are located in a single shaft, must be considered to provide proper relief and sequencing of door openings.

It can be helpful to interrupt stairs intermittently with well-sealed doors to minimize vertical airflow through buildings. This is particularly useful for fire stairs that extend through the entire height of the building. Entrances to fire stairs should be provided with good door and sill gaskets. (See the section on Door-Opening Forces under Pressurization System Design in Chapter 54 for guidance on ensuring doors in fire stairs can be opened during an emergency.)

Building air supply and pressurization systems should be configured in a maximum of 20- to 40-floor increments to facilitate effective building pressurization corresponding to building stack effect and wind pressure profiles.

The last key item is to ensure a tight exterior wall, which requires specification, proper testing, and hiring a qualified contractor to erect the wall.

The preceding precautions involve the architect and allied trades. The HVAC designer primarily must ensure that mechanical air-conditioning and ventilation systems supply more outdoor air than they exhaust, to pressurize the building above atmospheric pressure. This is true of all systems where a full air balance should be used for the entire building, with a minimum of 5% more outdoor air than the combination of spill and exhaust air provided at all operating conditions, to ensure positive pressurization. In addition, it is good design, and often required by code for smoke control, to have a separate system for the entrance lobby. Although not always required, this system can be designed to operate in extreme winter outdoor air conditions with 100% outdoor air. This air is used to pressurize the building lobby, which is a point of extreme vulnerability in minimizing stack effect. Lastly, if two-door vestibules must be provided, consider pressurizing the vestibule with conditioned outdoor air.

The world trend toward super- and megatall buildings suggests that both wind and stack effect will greatly affect designs of future tall building and HVAC systems. It is advisable to carry out both wind and stack effect analyses by computational fluid dynamic (CFD) or wind tunnel analysis during concept and schematic design phases of the project, such that advance precautionary measures could be implemented in the early design stage.

System designers typically apply safety factors at various points in the design process to avoid undersizing equipment. Judicious use of safety factors is good engineering practice. However, safety factors are too often misapplied as a substitute for engineering design, and this practice typically results in grossly oversized equipment. Therefore, care is necessary in applying safety factors.

Several alternative systems are used in tall buildings. Although the precise system configurations are subject to the experience and imagination of the design HVAC engineer, the most common ones are variations of generic all-air and air/water systems.

Unitary, refrigerant-based systems, such as through-the-wall units, are used in conjunction with all-air systems providing conditioned ventilation air from the interior zone, but this combined solution has been limited to retrofits of older buildings that were not previously air conditioned and smaller low-rise projects. They are seldom used in first-class tall commercial buildings.

Another option is panel-cooling-type systems, including chilled-ceiling and chilled-beam systems. Though not common in the United States, these systems are used in Europe as a retrofit alternative in existing buildings that were not previously air conditioned, because these systems can be installed with minimal effect on existing floor-to-ceiling dimension.

All-Air Variable-Air-Volume Systems. All-air variable-air-volume (VAV) systems in various configurations are one of the most common solutions in tall buildings. Conditioned air for VAV systems can be provided from a central fan room or from local floor-by-floor air conditioning units. These alternative means of delivering conditioned air are discussed in the section on Central Mechanical Equipment Room Versus Floor-by-Floor Fan Rooms. This section is primarily concerned with system functioning, configurations in use, and possible variations in system design.

VAV systems control space temperature by directly varying the quantity of cold supply air in response to the cooling load requirements. VAV terminals or boxes are available in many configurations; pressure-independent terminal units are recommended. Interior spaces that have a year-round cooling load regardless of outdoor air temperature can use any of the alternative types of VAV boxes:

The exterior zone can use any VAV box type, but in geographical locations requiring heat, the system must be designed with an auxiliary means of providing the necessary heating. This can be done by installing hot-water baseboard, controlled either directly by thermostat or by resetting the hot-water temperature inversely with the outdoor air temperature. Other alternatives are thermostatically controlled electric baseboard on the exterior wall, or either electric or hot-water heating coils in the perimeter VAV boxes.

Low-Temperature-Air VAV Systems. All of the preceding variations can be designed using conventional temperature differentials (16 and 20°F) between the supply air and room temperature. Buildings have been successfully designed, installed, and operated for decades with low-temperature supply air between 48 and 50°F. This increases the temperature supply differential to approximately 28°F, thus dramatically reducing primary air quantities and subsequently reducing air-handling system size and air duct distribution.

This lower-temperature air can be obtained by operating the refrigeration machines with chilled water leaving at 40°F or by using ice storage. If the chiller supplies 40°F chilled water, operating costs of the refrigeration plant increase and the chiller must operate for a longer time before an economizer cycle can occur. Moreover, use of absorption refrigeration machines may not be possible, because they usually cannot provide chilled water as cold as 40°F.

However, the reduced quantity of air distributed also reduces fan power, which more than offsets the additional energy used by the chiller. This lower-temperature air requires series-flow fan-powered VAV terminals or induction-type air supply terminals to mitigate draft and dumping concerns at the diffuser due to supplying low-temperature air directly to the space. The air delivery terminals mix room and cold supply air to deliver warmer air to the space to offset heat gain.

Using low-temperature supply air requires elimination of air leaks and proper installation of the correct thickness of duct insulation to prevent moisture condensation. Note that the decrease in supply duct size when using cold air can make lower floor-to-floor heights more practical.

Underfloor Air Distribution (UFAD) Systems. In underfloor air distribution (UFAD) systems, the space beneath a raised floor is used as a distribution plenum. Most installations use manually adjustable supply diffusers or automatically controlled terminal units beneath the floor to control air delivered to the space above. (In contrast, for more traditional systems, terminal units are installed above the ceiling and supply air is delivered from above.) When properly designed, either underfloor or ceiling-mounted air distribution systems can meet occupants’ comfort requirements. UFAD systems typically have a higher first cost because of the raised floor, but operating costs are usually lower because less fan power is required. However, if a raised floor is a design requirement for electrical distribution and information technology cabling, UFAD may offer savings in overall first and operating costs.

The UFAD system can use central fan rooms or floor-by-floor fan units. Conditioned air is typically provided at 60 to 64°F in the raised-floor plenum (between the structural slab and the raised floor), but in locations requiring dehumidification, the air must first be cooled to approximately 55°F to remove moisture and then blended with return air (often using an underfloor-mounted series fan-powered box or similar arrangement) to achieve supply air temperatures of 60 to 64°F. The suspended ceiling acts as a return plenum but can be reduced in depth because of the absence of supply ductwork.

A major concern with UFAD in tall buildings is the perimeter zone, which has widely varying loads between summer and winter conditions, especially in buildings with large glass exterior elements. Thermostatically controlled fan-coils beneath the floor or finned-tube radiation along the perimeter walls can be cost-effective solutions. Additionally, extreme caution is needed in sealing all structural floor penetrations to prevent short-circuiting of supply air.

Underfloor air conditioning for a tall building must be selected early in the design process, because it affects architectural (e.g., floor-to-floor heights, exterior facade treatment, stairs, elevators), structural (e.g., depressed structural slabs), and electrical (e.g., plenum-rated cabling) design considerations. All design disciplines must be involved in this decision process.

The combination of system components and the resultant system configuration for a specific building are limited only by the designer’s imagination. The chosen alternative is of interest and concern to the owner, architect, and other engineering consultants, and should be subjected to scrutiny and review by the entire design team before final selection is made.

Underfloor air-conditioning systems are a newer approach, where the space beneath the raised floor is used as a distribution plenum or where terminal units are installed beneath the raised floor (in contrast with more traditional systems, where the terminal units are installed above the ceiling). Either system, with ceiling-mounted terminals or one distributing air through the raised floor, when properly designed, will meet occupants’ comfort requirements. The underfloor air-conditioning system typically has higher first cost than comparable overhead distribution systems because of the cost of the raised-floor system. The cost premium can vary as a function of design details for the project, and can be substantially offset if the owner decides to incorporate a raised floor for power wiring and information technology cable distribution. Without this fundamental decision, the increase in the cost of the floor itself and a possible increase in the floor-to-floor height, with the resultant premium that must be paid for the exterior wall and the extended internal shafts, piping, and stairs, may be too great to justify the inclusion of the underfloor distribution system. Figure 8 shows a typical underfloor conditioning/ventilation system.

Multiple variations of underfloor air-conditioning system design are possible. Underfloor air distribution systems use the principle of displacement ventilation. Designs typically are implemented with all-air systems in which air is distributed beneath the floor, with the void between the slab and the raised floor serving as a supply air plenum. The conditioned air is provided at relatively elevated temperatures of approximately 60 to 64°F by blending cold, dehumidified supply air with warm return air. This air then passes at low velocities from the air-conditioned floor through floor outlets and rises vertically to the ceiling through its own buoyancy, removing heat from occupants, office equipment, and lighting as it rises. The ceiling and the space above it function as a return air plenum where distributed air is collected and returns to the air-conditioning supply system, which can be either a central or floor-by-floor system. Because supply ductwork is not needed, the plenum above the ceiling can be reduced in depth compared to that required for an overhead distribution system.

Figure 8. Typical UFAD System

A variation of the underfloor air-conditioning system is using all-air terminals or fan-coil units beneath the floor in the exterior zone. A thermostatically controlled terminal can be advantageous in altering unit capacity in the exterior zone with its widely varying loads. In addition, using a fan-coil unit, which can modify its capacity output as the load varies and has an inherently greater capacity on a percent basis than an all-air terminal, may provide a more cost-effective solution for tall commercial buildings, particularly those with larger glass elements in the exterior wall. The design using fan-coil units is the same as with all-air terminal designs: air is distributed through floor grilles, with the ceiling acting as a return air plenum.

Many commercial and office projects in Europe include a raised floor for power wiring and information technology cabling, so underfloor distribution systems have been widely accepted throughout the continent. These systems have found more limited application in the United States, probably because raised floors are used infrequently and the National Electric Code^® (NFPA Standard 70) requires that all cabling in an air plenum must be installed in conduit or carry a plenum rating if the raised floor is used for free discharge of supply air. (Where a raised floor is used for cable distribution only, conduit or plenum-rated cabling is not required.) This can increase the cost of cabling significantly and can therefore be a significant consideration in the decision process.

Underfloor distribution systems using variable-air-volume or fan-coil terminals are applied more widely. These systems have a lower space reconfiguration cost as occupancy changes, because all that is required is relocation of a floor diffuser to meet the altered space needs (akin to relocation of an electrical outlet to serve a new occupant layout). This lower cost of interior modifications should be fully considered by the owner and the design team.

Floor supply systems that mix with the total air mass in the occupied zone are not displacement systems. Displacement systems result in temperature gradients in the occupied space, whereas fully mixed systems minimize room temperature gradients.

The displacement system effectively delivers supply air to those parts of the space where heat gain occurs and not the whole occupied volume, so less supply air should be needed.

Fully mixed floor supply systems can handle spaces with high heat gains (>30 Btu/h · ft²), and have considerably greater capacity than displacement systems alone (~10 Btu/h · ft²). The floor supply system creates zones of discomfort near the outlet, between 3 and 5 ft radius, where sedentary occupants should not be located. There is a relatively low air volume per outlet compared with high-level diffuser systems, which require the use of more supply outlets.

Because the air supply stream is delivered directly into the occupied zone, supply velocity and temperature are restricted, limiting maximum sensible cooling load to 12.64 Btu/h · ft² for a 9.8 ft high floor to ceiling height; higher loads can be handled where the floor-to-ceiling height is greater.

Use great caution with floor-to-ceiling heights less than 9.8 ft, because the higher temperatures developed at the ceiling may cause uncomfortable radiant effects. System performance improves with ceiling height.

Consider using exhaust air heat recovery. Recirculation of room air should be minimized, because this air will be hot and vitiated, generally with a higher specific enthalpy than outdoor air.

If air patterns in the space are subject to considerable disruption (e.g., by occupant movement or high infiltration rates), system effectiveness will be reduced.

A displacement ventilation system should not be used for heating because the low-velocity heated air makes effective air distribution very difficult. A separate perimeter heating system should be provided.

Selection of supply outlets should be based on minimizing the zone of discomfort around the supply outlet; this entails using more small outlets rather than fewer large ones. The geometry of the supply outlet is not as critical as that for diffusers and registers used in conventional mixing systems.

Match the supply volume flow to the volume flow rate of the plumes set up by internal heat sources at the given boundary height.

The height of the boundary plane depends on supply air volume: it will be higher if excessive air is delivered, and lower if supply air is insufficient.

Chilled beams are a type of air/water system that have had increasing success in tall buildings. These units are available in both passive and active types, with active units offering higher capacity. Passive chilled-beam units rely on a combination of radiant and convective heat transfer to provide space conditioning from heated or chilled water delivered to the unit. With active units, primary supply air delivered to the unit causes induced room air to circulate through a hot- or chilled-water coil to provide additional conditioning capacity.

Chilled beams allow an overall reduction in the ductwork required to condition the space, because water has a greater heat-carrying capacity than air. Consequently, sheet metal costs and potentially space requirements for supply and return air ductwork can also be reduced. Use caution, however, because chilled-beam units have no condensate drain and should be designed without latent cooling capacity, so the primary supply air must be conditioned to deliver air at a low enough dew point to provide the required dehumidification of the space served.

Radiant cooling follows the same principles as radiant heating: heat transfer occurs between the space and the panels through a temperature differential. However, unlike in radiant heating, the colder ceiling absorbs thermal energy radiating from people and their surroundings. The major difference between cooled ceilings and air cooling is the heat transport mechanism. Air cooling uses convection only, whereas cooled ceilings use a combination of radiation and convection. The amount of radiative heat transfer can be as high as 55%; convection accounts for the remainder. With cold ceilings, the radiative heat transfer occurs through a net emission of electromagnetic waves from the warm occupants and their surroundings to the cool ceiling. On the other hand, convection first cools the room air because of contact with the cold ceiling, creating convection currents in the space, which transfers the heat from its source to the ceiling, where it is absorbed.

Because air quality must be maintained and radiant panels remove only sensible heat from the space, radiant cooling panels are used in conjunction with a small ventilation system. The panels provide most of the sensible cooling, and the air system provides ventilation and air moisture (latent load) control. To prevent high humidity levels in a room, the supply air must be drier than that of the supplied space, especially when there are additional moisture sources in the room. Consequently, outdoor air must be dehumidified, which is usually done by cooling to a dew point of approximately 59°F. If the environment is dry, the ventilation system is used to humidify the air. Because the ventilation system is used only to maintain the air quality and to regulate the latent load, the airflow required is small relative to conventional cooling systems. Best results are usually attained with a straight displacement ventilation system with no air recirculation. This system typically supplies air through outlets near or at the floor, at temperatures below that of the room air; this approach provides a uniform layer of fresh air at floor level. In turn, people and other heat sources create a passive convective flow of fresh air to the ceilings, where it can be exhausted. This reduced airflow and radiant panels’ relatively high surface operating temperature (mean temperature of 61°F) make radiant cooling a more comfortable way of cooling a space than conventional systems.

A cooled ceiling operates in direct proportion to the heat load in the room. Typically, a person sitting at a desk emits 445 Btu/h of energy, whereas a computer emits 300 to 1800 Btu/h to its surroundings. The radiant panel capacity should be determined by the operation conditions (water temperature and flow) and the space temperature. The greater the number of people and/or appliances and exposure to sunlight, the greater the space heat load (and therefore greater increased capacity of the cool ceiling). Generally, cool ceilings can handle between 10 and 20 W/ft² with up to 50% of the ceiling space used for cooling.

Condensation on the surface of the panels is not a problem with radiant cooling as long as the supply water temperature is properly controlled. Because condensation of water occurs when the panel temperature reaches the space dew-point temperature, proper water temperature control helps avoid condensation. The space dew-point temperature should be monitored by a sensor linked to a controller, which modulates the inlet water temperature accordingly. Therefore, if there is risk of condensation, the water temperature is raised or water flow is shut off. However, the lower the panel’s inlet temperature is, the more work the panels do; the inlet temperature should be at least 2°F above the room’s dew-point temperature. Consequently, the cooling capacity of a radiant cooling system is generally limited by the minimum allowable temperature of the inlet water relative to the dew-point temperature of the room air.

Fan-coil units, either vertical stacked or horizontal, are often used in tall hospitality or residential buildings. Built-in variable-frequency drives provide an energy advantage to the overall building energy consumption, as well as improving temperature control in spaces conditioned by these units. For details, see the section on Fan-Coil Unit Systems in Chapter 20 of the 2020 ASHRAE Handbook—HVAC Systems and Equipment.

These units can be either complete with cabinets (which can be exposed in the space) or built into the general construction (less obtrusive to building aesthetics). Vertical units are even available with vertical pipe risers factory installed, reducing field-installed piping and overall construction costs. Although these internal components are generally designed and tested for elevated pressure capabilities, the actual pressure on these components for a particular building height should be investigated.

VRF systems for heating and cooling are becoming more prevalent for reducing energy consumption in space conditioning. This system option is viable for use in a tall building, particularly the newly available water-cooled condensing unit option. Air-cooled conditioning may also be viable, but requires significant amounts of space outside the building, and tall buildings typically have small roof areas and limited space on the ground. In addition, the refrigerant lift available from these units is limited, which typically makes air-cooled options less desirable. For details, see Chapter 18 of the 2020 ASHRAE Handbook—HVAC Systems and Equipment.

In central fan rooms, the supply of conditioned air for each office floor originates from multiple air-handling systems located in one or more central fan room(s), which are frequently identified as central mechanical equipment rooms (MERs). Each air-handling system can be provided with an outdoor air economizer through minimum and variable outdoor air dampers, as dictated by the annual ambient temperature and humidity conditions and building code requirements. Multiple systems in a fan room can be interconnected by delivering supply air into a common discharge plenum from all supply systems on that floor.

Air from the central fan room(s) is distributed to each floor by means of vertical duct risers in fire-rated shafts (typically 2 h rated) within the core of the building. At each floor, horizontal duct taps are made into each riser. This horizontal duct tap contains a fire damper or a fire/smoke damper, as required by the local building code, that must be installed where the supply air duct exits the rated shaft enclosure. In many situations, an automatic, remotely controlled two-position damper, which can be rated as a smoke damper, provides individual-floor overtime operation and smoke control. The position (open or closed) is typically controlled by the building management system either on an occupancy schedule or by occupancy sensor or manual reset switch.

Return air from each floor’s ceiling plenum also enters the vertical shaft though a return air fire damper at each floor.

Return air is often not ducted within the shaft, so the air is carried back to the central fan room in the 2 h rated drywall shaft. In each central fan room, multiple return air fans draw return air from the return air shafts and deliver it to a headered return air duct system in the central room and then to each air-handling unit.

With an outdoor air economizer, return air is either returned to the supply air system or exhausted to atmosphere, as determined by the relative dry-bulb temperature (or enthalpy) of the return air and the outdoor air being provided to the building. Quantities of outdoor and return air depend on the season and the resultant outdoor temperature and humidity. In warmer climates where the systems operate on minimum outdoor air at all times, return air is always returned to the supply air system except during morning start-up or where the fans are operating in smoke-control mode.

A typical central fan room and supply and return air shaft arrangements are shown in Figure 10.

The air supply for each office floor under this alternative originates from a local floor fan room, typically located in the building core. This room contains a chilled-water air-handling unit with a cooling coil, filters, and fan(s). Morning heating at start-up in cold climates can be provided by a heating coil in the air-handling unit, a unit heater installed in the local fan room, or heating coils in the VAV or fan-powered VAV (FPVAV) boxes. The unit on a given floor usually only supplies the floor on which the unit is installed. Typically, one unit is installed on each floor, but multiple units may be used with interconnected air systems on large floors. Chilled water for the cooling coil is provided by a central chilled-water plant in the building, sized to meet the combined capacity requirements of all of the cooling and heating needs. The supply air fan in the air-conditioning system both supplies air and returns it from the zone served. Return air is typically directed to the fan room through the ceiling plenum, but may be either ducted or unducted in the fan room. In most cases, however, the fan room acts as a return air plenum.

Figure 10. Central Fan Room Arrangement

This system typically operates on minimum outdoor air during all periods of occupancy. Outdoor air for the system is provided by an air-handling unit serving as a dedicated outdoor air system (DOAS), located on the roof or in a central mechanical equipment room. This unit provides conditioned outdoor air to the unit on each floor by a vertical air riser routed to each air-handling unit. The outdoor air unit may include preheat and cooling coils to treat incoming outdoor air, and should contain filtration to clean this air. This unit can contain heat recovery to precondition the outdoor air by recovering heat or cool from exhaust air, which may be required by the applicable energy code.

Although chilled water is typically provided by a central refrigeration plant, economizer requirements can be provided by cooling the chilled water in mild weather by condenser water from the cooling tower. During periods of low wet-bulb temperature, the condenser water cools the chilled water through a heat exchanger in the central chilled-water plant or by refrigerant migration through the refrigeration unit.

A typical local fan room supply, return, and outdoor air arrangement is shown in Figure 11. The unit heater shown provides morning heat. It can use electric energy or hot water as its heat source.

As shown in Figure 11, the walls around the local floor fan room are not fire rated because the duct penetration serves only this floor. The vertical shaft that contains the outdoor air duct from the central fan room, and perhaps the smoke exhaust ducts, constitute a fire-rated shaft. Accordingly, fire dampers are only provided at the point where ducts penetrate the shaft wall, not as they leave or enter the local floor fan room itself. Although fire dampers are shown in the smoke exhaust ducts, many codes prohibit their use in an engineered smoke control system to avoid the possibility of having a closed damper when smoke removal is required.

A variation of the floor-by-floor alternative consists of a floor-by-floor air-conditioning supply system that is virtually identical to that in the chilled-water alternative. In this alternative, a packaged, self-contained, water-cooled direct-expansion (DX) unit, complete with one or more refrigeration compressors and water-cooled condensers, is used to produce the cooling. The heat of rejection from the compressor is handled by a circulating condenser water system and cooling tower. If geographic location dictates an economizer, this need can be met by a free-cooling coil installed in the packaged unit that will only operate when condenser water delivered to the unit is cold enough to provide effective cooling. The only central cooling equipment is a cooling tower, condenser water pumps, and the central outdoor air supply unit. If an open tower system is used, consider providing a way to remove particulates from the circulating condenser water. Depending on the size of anticipated particles, typical options include sand filtration, media filtration, and centrifugal separators. For an open system, condensers should be cleanable. Bear in mind that significant water will end up on the floor during condenser cleaning, so it is important to ensure that the room has a recessed floor drain and that the floor is moisture sealed.

The physical arrangement of the supply air unit does not differ from that shown in Figure 11, except that the chilled-water risers are replaced by condenser water piping.

A popular variant location for a packaged floor-by-floor unit is on an outer wall. This location obviates the need for a separate outdoor air unit in a central fan room. Outdoor air can be directly introduced to the floor-by-floor unit through a louver and automatic louver damper for each unit. Moreover, this arrangement may allow using an air-cooled condenser to handle heat of rejection. If the location requires an economizer, include a minimum and variable air damper behind the outdoor air louver.

Figure 11. Floor-By-Floor Air-Conditioning Unit Layout (Normal Operation)

Several precautions are necessary. If an outdoor air economizer is used, the return air spill damper must be located carefully to ensure that outdoor air and spill air do not mix. Similar care must be taken to avoid air-cooled condenser intake air mixing with air previously spilled to atmosphere. There must be no possibility of mixing heated discharge air with either the condenser intake air or the outdoor ventilation air for the supply air-conditioning unit. This can become a complicated arrangement, which may necessitate locating the air-cooled condenser remote from the local fan room.

An accurate comparison of alternative schemes can only be made with a developed set of schematic plans in sufficient detail to allow a cost estimate to be completed by the contracting team or a professional estimating service. For an example, see Table 2.

Acoustical criteria should be established for the various types of occupancy that are expected in the building. For example, open-plan office space can be designed to meet a noise criteria level of NC-40, whereas private and executive offices or conference rooms should be no higher than NC-35, and may be required to be even lower. The acoustical engineer on a project sets these levels, and it is the responsibility of the HVAC designer to work with the acoustician to see that the criteria established are achieved in the final installation. (For details on sound levels, see Chapter 49 in this volume and Chapter 8 in the 2017 ASHRAE Handbook—Fundamentals).

Equipment and system selection affects the required sound treatment and resultant noise levels in occupied areas. It is important that project acoustical standards and the final design are reviewed by the acoustical consultant to ensure that the desired noise levels can be achieved, particularly when floor-by-floor fan rooms are used.

Detailed analysis is needed to determine the cooling method that should be installed in a project. The choices are usually limited to either centrifugal refrigeration or absorption chilled-water machines, although recent developments have made screw chillers more relevant for use in tall buildings. Centrifugal machines can be electric drive or steam drive; screw machines are available only with electric motor drives, and both are almost always water-cooled. Absorption machines can be single or double effect, but the latter require high-pressure steam to achieve their lower energy costs. High-pressure steam is rare in today’s commercial projects unless the steam is available from a central utility.

Air-cooled refrigeration machines have been installed in tall buildings, but infrequently: commercially available sizes of air-cooled refrigeration equipment are limited, and space requirements are comparatively excessive. The largest air-cooled refrigeration machine that currently can be purchased this time is approximately 400 tons. Tall buildings, by nature, are typically large, and the number of air-cooled refrigeration machines and relatively large equipment space that would be required usually make air cooling not viable. In addition, air-cooled equipment’s operating costs may be higher because of higher condensing temperatures developed by the refrigeration equipment caused by outdoor dry-bulb temperatures that are higher than the coincident wet-bulb temperature. Water-cooled equipment’s refrigerant condensing temperature, on the other hand, is driven by the lower outdoor air wet-bulb temperature. This operating cost difference exists even though there is no cooling tower fan or condenser water pump.

Air-cooled equipment may, however, find application in tall buildings where water for cooling tower makeup either is not available or is prohibitively expensive.

For tall buildings that do not use electric resistance heat, the fuel-fired heating plant includes boilers fired by oil or gas, by both fuels (with oil as a standby fuel), or by electricity. These boilers provide hydronic heat and low-pressure steam for distribution to spaces in the building, or act as supplements to heat pumps or heat recovery systems. Choosing the correct solution for a building is subject to an economic analysis that considers space requirements, first cost, and operating expense.

Further complicating the energy transfer source decision is the location of the equipment within the building. This affects structural costs, architectural design, construction time, and availability of cooling or heating relative to the initial occupancy schedule. A below-grade location could potentially provide early heating availability, but also could complicate the design process and result in higher overall project costs. Locating cooling and heating plants on floors above grade, up to and including space immediately below the roof, is common and may be desirable for simplicity of construction and ease of providing the necessary ventilation air and other services to the equipment. Moreover, the two types of plants need not be installed at the same level in the building, because there is usually no direct interconnection of the two plants.

Virtually any location in a tall building can be used for the heating and cooling equipment. When choosing the location, consider the following:

Considerations for the refrigeration plant location are more complex. Not only must electricity, gas, oil, or steam be brought to the machine to operate the equipment, but chilled and condenser water also must be pumped from the refrigeration plant to the air-conditioning supply equipment. In addition, the cooling tower and the working pressure of the refrigeration machines, piping, fittings, and valves must be reviewed based on the static height of liquid above this equipment, as discussed in Ross (2004) and Chapter 40 of the 2020 ASHRAE Handbook—HVAC Systems and Equipment.

Because of the world trend of increasing building height and because tall buildings tend to be mixed use, split chiller and boiler plants are becoming more common: part of the heating and cooling production plant is located at the top part of the building, and part at the basement. The reason for this split is to limit pressure on both halves of the plant distribution systems.

In addition, installing a cogeneration or trigeneration plant in tall buildings to meet green and sustainability initiatives is becoming popular. These facilities use engine- or turbine-driven generators to deliver electricity to the building and generate either chilled water through absorption chilling and/or heating water.

Acoustics and vibration also are key considerations during architectural, structural, and mechanical design. The HVAC designer and project acoustician should place mechanical equipment to achieve the desired acoustical levels in spaces above, below, or adjacent to the central plant. Achieving the proper solution involves understanding the characteristics of sound generated by the equipment and the various paths (e.g., through floors, ceilings, walls, building structure) for transmission of that noise and vibration to occupied areas of the building.

Regardless of the type of equipment being installed on a project, it is prudent to specify a maximum permissible sound level for equipment. Sound and vibration generation, transmission, and correction are discussed in Chapter 49 in this volume and in Chapter 8 of the 2017 ASHRAE Handbook—Fundamentals.

The locations of the boiler and chiller plant also affect the construction schedule. This concern is especially critical for the refrigeration plant, which is a complex installation that involves a significant amount of labor because of the need to complete the chilled-water, condenser water, and possible steam piping as well as provide for the electrical capacity requirements of the machines. The heaviest piping and most difficult installation process for piping in the building occur at the refrigeration plant. As a result, if the refrigeration plant is on the uppermost level of the building, installation of the machines and their associated piping can delay the overall schedule. Accordingly, if the refrigeration equipment cannot be installed in the below-grade level because that space has other priorities (e.g., parking, storage), the refrigeration plant may be best located above the lobby level and below the uppermost levels of the building.

A major consideration in piping system design for a tall building is the hydrostatic pressure created by the height of the building. This hydrostatic pressure affects not only the piping and its associated valves and fittings, but also equipment in the building; in the chilled-water system, this includes refrigeration machines, casings for chilled-water pumps, cooling coils in air-conditioning systems, heat exchangers, and any fan-coil units at the exterior wall of the building. A similar list of devices beyond piping, valves, and fittings can be developed for other pumped systems such as the condenser water or any hot-water system.

Dynamic pressures created by the pumps also must be added to the static pressure to determine the working pressure on any element in the piping system. This dynamic pressure is the total of the following elements:

The working pressure of the piping and connected equipment at various elevations in the building must be known. This is found by adding the hydrostatic pressure at the specific location to the dynamic pressure that can be developed by the pumps at that location. The dynamic pressure at any point should include the pump pressure at or close to pump shutoff at full speed, even if variable-speed pumps are used, because it is possible for the pumps to operate at this shutoff point in the event of a VFD failure. This working pressure on piping and equipment invariably lessens as the static pressure at a specific location is reduced.

The trend of ever-greater height makes piping system hydrostatic pressure zoning design very important for both technical and economical reasons. Check the pressure rating of all major air-conditioning equipment to confirm whether the required pressure-rated equipment is economically available in the market.

The level on which the refrigeration machines and the supporting chilled- and condenser water pumps are located in a building can affect the cost of refrigeration equipment, the pumps, the piping, and the fittings and valves associated with the piping. There is economic impact because of the working pressure to which the equipment, piping, fittings and valves will be subjected by the height of the system above.

Using the following information, calculate the effect of alternative chiller locations in a tall building: at basement level, a midlevel mechanical equipment room, and a mechanical equipment room on the roof. There would be an open expansion tank at the top of the building (the highest point in the system) in all three alternatives. If a closed expansion tank is used, the maximum pressure must be established and considered in the determination of the system’s working pressure.

The standard working pressure for coolers and condensers on large refrigeration machines from all of the major manufacturers in the United States is 150 psig. These machines can be manufactured for any working pressure above 150 psig for additional cost. The incremental increase in the cost of a given vessel becomes larger with each unit of increase in the working pressure. Accordingly, it is necessary for the HVAC design engineer to accurately determine and separately specify the working pressure on both the cooler and the condenser of the refrigeration machines.

Working pressure on the refrigeration machine can be reduced by locating the chilled-water pump on the discharge side rather than the suction side. If this is done, the residual pump pressure on the refrigeration machine water boxes is reduced to the sum of the hydrostatic pressure and this nominal value of dynamic pressure from the pumps. This can reduce the cost of the refrigeration machines, but does not alter the pressure on the pump casing and flanges, which must still be the sum of the static and dynamic pressures.

Pressure on (and cost of) refrigeration equipment can be reduced by locating it above the basement; this, however, will not alter the maximum pressure experienced by the pipe, fittings, and valves at any location that is used. It is possible, however, to reduce the chilled-water working pressure on both the machines and piping by using plate-and-frame heat exchangers, which segregate groups of floors into separate static pressure zones.

In the 900 ft tall example building with the refrigeration machine in the basement, it is possible to break the chilled-water system into three separate zones (Figure 12).

Figure 12. Typical Chilled-Water Distribution System for Supertall or Megatall Building

Each zone has static pressure of one-third of the total building height, or 300 ft. All of the pumps are located on the discharge side of the refrigeration machines or the secondary zone heat exchangers. The result is that the maximum head of each zone is 143 psig, which is below the threshold design pressure of 150 psig, or the point at which an increased pressure rating for the chiller and other heat transfer equipment must be considered.

The working pressure of the primary chilled-water pump in the basement will not change substantially from that required where no secondary systems were included, because the primary chilled-water pump must now overcome the loss through the flat-plate heat exchanger. In addition, motor-driven pumps are added at each secondary water heat exchanger. Finally, with the two additional zones and the resultant chilled-water temperature increase, there is a requisite increase in the volume of water flowing through the systems on the upper floors. Accordingly, although there are benefits in the reduction in pressure, there are partially offsetting considerations that must be analyzed to determine the overall cost effectiveness of using flat-plate heat exchangers to reduce the operating pressure on the equipment, pipe, valves, and fittings at a given level.

Use of flat-plate heat exchangers and their location in a chilled-water piping system is subject to an economic analysis by the design HVAC engineer to determine the first cost of alternative arrangements as well as the operating cost differentials, if any, for any scheme.

Using a flat-plate heat exchanger to reduce working pressure on the condenser, although feasible, is not often considered, because the condenser water piping is usually in a single shaft with minimal (if any) offsets and a resultant small number of fittings. Valves are also only installed at the machines and are few in number. This limited number of fittings and valves may not be sufficient to offset the cost of the flat-plate heat exchanger and its valving as well as the added pump on the secondary side of the heat exchanger. Beyond that, there is an increase in the temperature of the condenser water, which increases the cost of operating the refrigeration machines.

The working pressure on the piping, valves, and fittings at various levels in a building must be determined so that proper piping material can be specified. In the United States, with steel pipe, Schedule 40 pipe is the standard wall thickness for pipes up to 10 in. diameter. For pipes 12 in. and larger, the pipe standard that is used has a wall thickness of 0.375 in. Either of these standards would accommodate the working pressures experienced in any expected pipe diameter in any tall building. The allowable pressures for various pipe diameters can be found in ASME Standard A17.1 and the Boiler and Pressure Vessel Code (ASME 2013) and in the publications of various pipe manufacturers. The valves used should be reviewed in the valve manufacturers’ literature to ensure their ability to meet the project’s requirements.

For steam condensate piping or for condenser water piping, where corrosion is a possible concern, pipe with a heavier wall thickness should be considered, although not because of the working pressure on either system.

Piping materials other than steel are often used. For pipe sizes below about 4 in., in the cases of runouts or in open condenser water piping where corrosion is a concern, copper is the usual choice. Copper pipe is rare, but copper tubing is common. The limiting factor in the use of copper tubing is usually at the joints, where the ability to handle higher working pressure is restricted.

Other factors piping design should consider include

In addition to expansion and contraction of the piping caused by changes in the ambient temperature or of the pumped fluid in the piping, frame shortening can be a problem in concrete buildings. Concrete shrinks as it cures: over time, this shortening can be in the range of 1/8 in. per floor. Although this movement is relatively small, it amounts to about 9 in. for a 70-story building. This condition requires that pipes above, below, and between anchor points be flexible enough to allow for pipe movement with respect to the structure. To properly design for this condition, the HVAC designer should obtain from the structural engineer the exact amount of movement that the piping system can experience.

Traditionally, rules of thumb for selecting refrigeration machines in the United States have used a 10 or 12°F temperature differential between entering and leaving water in the chiller and a 10°F differential or 3 gpm per ton of capacity for the condenser. These guidelines are appropriate for small buildings, because they have little effect on project cost, but may be less ideal for large buildings, particularly tall buildings. In projects of this type, the capital costs of piping, valves, and fittings can be substantially reduced, with a possible penalty in refrigeration machine operating cost, by using larger temperature differentials with lower water flow and a consequent reduction in piping diameter.

For a large project with a total cooling capacity requirement of 4000 tons and chilled-water flow at a 10°F temperature differential, 9600 gpm is circulated through 20 in. piping at approximately 10 fps. If a 16°F temperature differential is used, total flow from the refrigeration plant is 6000 gpm and the piping is 16 in. Cost savings on the piping using the greater temperature differential would be significant. Also, although the kilowatts per unit of cooling under both conditions should be studied, with the same discharge temperature, the operating energy consumption probably is unchanged.

For the 4000 ton refrigeration plant with a 10°F differential, the condenser water flow is 12,000 gpm and 24 in. piping is required. If this temperature differential were increased to 15°F, condenser water would be reduced to 8000 gpm, and the piping to 20 in. Again, this change results in a significant first-cost savings, depending on the distance between the refrigeration machines and the cooling towers.

Energy consumption for the refrigeration machines might marginally increase, because the condensing temperature of the refrigerant and the resultant energy usage is largely (but not solely) a function of the leaving condenser water temperature. Increases in chiller energy consumption may be partly or fully offset by reduced pumping energy. Furthermore, large chiller plants designed to maximize chilled- and condenser-water temperature differentials (lower flow and smaller piping) can offer substantial savings in piping system installation cost.

The elevator machine room’s cooling loads consist not only of the electric motor that drives the hoisting mechanism but also of extensive heat-generating electronic elevator controls. The electronic components that are part of the system require that the elevator machine room be maintained at a temperature between 80 and 60°F. This can be accomplished by means of a packaged DX condenser water-cooled unit in the elevator machine room; however, because of possible significant operational availability restrictions on the use of water in the machine room, the HVAC designer should review this alternative with the building developer and possibly code officials. Using a packaged DX condenser water unit may be necessary for a low- or mid-rise elevator bank with its machine room in the middle of the building, without easy access to outdoor air unless the remainder of the floor is used as a mechanical equipment room. At the top of the building, the cooling equipment can be air cooled.

The ultimate size of DX units is determined by information provided by the elevator manufacturer. The elevator consultant can provide the necessary general information to allow the design to proceed through bidding. The amount of cooling for this equipment can be significant: as much as 10 to 15 tons for a single elevator equipment room.

All elevators installed in the United States must conform to ASME Standard A17.1, as modified by local authority and applicable building code. One requirement of many codes is to include a vent opening at the top of each elevator shaft that is 3.5% of the plan area of the hoistway or 3 ft² per elevator, whichever is greater. The purpose of this requirement is to allow venting of smoke during a building fire. To accomplish this, a duct must be provided from the vent to atmosphere. This is simple at the top of the building, but for low- and mid-rise elevators, where the elevator equipment room is not located in a mechanical room with perimeter access, extending the connecting duct to atmosphere may be difficult.

Under many codes, including the model International Building Code^® (IBC [ICC 2015]), for a building that is fully sprinklered, the need for the vent and its extension to atmosphere may be waived for passenger elevators, except for buildings where there is overnight sleeping (e.g., hotels, residences). The vent is typically still required for a dedicated service elevator car.

In addition, under the IBC, the vent may be closed under normal building operating conditions by including an automatic damper in the atmospheric vent or, under some code jurisdictions, by installing a piece of glass that will break in a fire. This damper must open on detection of smoke by any of the elevator lobby smoke detectors. Dampers have a distinct advantage in that they are manually and remotely resettable.

Where elevator speeds are greater than 1400 fpm, vents at the bottom of the shafts may be required by code to allow rapid escape of air when the high-speed car is descending.

In super- and megatall buildings, express (or shuttle) elevators are provided to quickly carry occupants to upper-level occupancies, typically in hotels or residential uses. These elevators are commonly used as evacuation elevators in emergency situations. To maintain the safety of these elevators for this use, the elevator shaft(s) should be pressurized to keep the shaft and cars free of smoke. Refer to the section on Smoke Management for more details.

If part of the chilled-water or boiler plant is located in the top zone of the supertall buildings, the freight elevator should have sufficient capacity and cab size to deliver and transfer all major equipment from the ground level to the area where the upper plant is located, to aid in maintenance of the equipment located there.

In the United States, the International Building Code^® (IBC) is the predominant building code; in Canada, it is the National Building Code of Canada (NRC 2010). The National Fire Protection Association’s (NFPA) Standard 5000 generally incorporates NFPA Standard 101. These codes do not define a “tall building,” but have additional requirements for a high-rise building greater than or equal to 420 ft in height.

Tall buildings share many of the code requirements of other high-rise buildings. The IBC (ICC 2018) defines a high-rise building as “a building with an occupied floor located more than 75 ft above the lowest level of fire department vehicle access.” Additional requirements are imposed for buildings 120 and 420 ft above grade. No specific definition of “tall building” is contained in the codes.

Key fire safety provisions for tall buildings should include the following:

Automatic smoke detection should be provided in elevator lobbies, elevator machine rooms, mechanical and electrical equipment rooms, and any other spaces not provided with automatic sprinklers. The detection system should be connected to the automatic fire alarm system. Duct smoke detectors should be provided in the main return air and exhaust air plenum of each air-conditioning system with a capacity greater than 2000 cfm. Duct smoke detectors are also needed at each connection to a vertical duct or riser serving two or more floors from a return air duct or plenum.

The smoke detection system should be designed in accordance with NFPA Standard 72.

Residential buildings should have smoke alarms in each room used for sleeping purposes and on the ceiling or wall outside of each separate sleeping area. The smoke alarms should be interconnected so that activation of any smoke alarm in the dwelling unit activates all of the smoke alarms in that unit. This does not require activating smoke alarms in other apartments in the building.

Complete automatic sprinkler protection should be provided in accordance with NFPA Standard 13.

Standpipe systems should be provided in accordance with NFPA Standard 14.

The essential features of smoke management design are described in Chapter 54. Additional information is contained in NFPA Standard 92A.

The IBC requires exit stairs to be smoke protected. One way to achieve this is with a smokeproof tower of pressurized stairs. To enhance egress for buildings 420 ft high or more, the codes require either an additional exit stairway beyond those required by the typical exit calculations, or pressurization of the elevator shafts. To prevent smoke spread through the elevator without elevator shaft pressurization, elevator vestibules with a minimum 1 h fire resistance rating are required.

Codes also require an elevator for use by emergency responders, with access from a vestibule directly connected to an egress stair.

Elevators to be used for occupants in an emergency require special protection, including pressurized elevator shafts, an emergency voice/alarm communication system, elevator lobbies with direct access to a exit enclosure, and a means to protect the elevator from automatic sprinkler system water infiltrating the hoistway enclosure. Automatic sprinklers are prohibited from the elevator machine room, and shunt trips for elevators shutdown should not be provided.

All life safety systems are required to have standby power designed and installed in accordance with NFPA Standards 110 and 111, as appropriate.

A fire command center is required in a protected location at or near grade to monitor all fire safety and emergency systems. It should also have controls for the smoke management system and emergency power system.